Ovarian cancers are heterogeneous and the most deadly histotype, called high-grade serous cancer (HGSC), is typically diagnosed in the late stages. One potential contributing factor to late diagnosis is a gap in knowledge regarding the step-wise progression of the disease. In the last several years, the source of HGSC has been challenged, and now that the secretory epithelial cells of the distal fallopian tube fimbria are the focus of research describing pre-malignant changes. In women with a mutation in BRCA1/2, prophylactic removal of the fallopian tube fimbria revealed that one of the earliest reported changes in fallopian tubal epithelium (FTE) is the loss of a protein called Pax2. Fallopian tube secretory cells that lack Pax2 protein expression have been termed secretory cell outgrowths (SCOUTs). SCOUTs are thought to progress to ?p53 signatures,? upon mutation in the p53 gene, and ultimately these benign lesions progress to serous tubal intraepithelial tumors (STICs) and high-grade serous tumors. Very little is known about what regulates Pax2 expression in the FTE, the functional significance of its loss in the FTE, and whether re-expression of Pax2 in HGSC may slow or halt tumor growth. The NCI Provocative Question #1 is specifically interested in understanding transitions between pre-malignant and malignant populations of cells. For ovarian cancer this could not be more important as the survival rate of ovarian cancer patients when detected early are 95%, but almost 75% are detected late stage where five year survival rates are less than 25%. We have developed novel model systems of otherwise normal FTE that can be studied to understand how loss of Pax2 contributes to malignancy. Since SCOUTs and p53 signatures are considered benign, reversal of these precursor lesions may provide a method to potentially prevent them from progressing to cancer. In addition, forced expression of Pax2 in HGSC cell lines resulted in less migration and proliferation. We plan to develop a system to screen small molecules that facilitate expression of Pax2 as a potential drug therapy. Overall this grant will determine if Pax2 protein expression blocks tumor formation (Aim 1), its downstream targets that are critical for initiating tumor formation (Aim 2), and strategies to enhance its expression in tumors (Aim 3). These studies will provide critical information regarding the step-wise progression of preneoplastic lesions into tumors. Our proposal will also attempt to exploit the characteristics of early tumor formation, loss of Pax2, as a therapy by searching for compounds that can stimulate Pax2 re-expression.